1. Field of the Invention
The present invention relates to methods for scanning a two-dimensional surface of one or more objects and, more particularly, to an apparatus and method for optimally scanning, for example, the surface of one or more semiconductor wafers undergoing a chemical etching process.
2. Description of the Prior Art
In one particular application wherein the present invention is especially practical, a silicon-on-insulator (SOI) semiconductor wafer undergoes a chemical etching process. During this process, a chemical etching instrument induces a removal of material from a surface of the SOI semiconductor wafer. The material is removed as a result of a chemical reaction that occurs between the surface material of the SOI wafer and a plasma that is generated by the chemical etching instrument. Such an instrument is described in a related patent application entitled, System for Removing Material from a Wafer, U.S. patent application Ser. No. 07/696,897, filed on May 7, 1991 U.S. Pat. No. 5,254,830. As described in this system, a chemical etching instrument probe is scanned over the surface of a wafer in a controlled pattern. While scanning the wafer surface, the probe emits a plasma that induces a constant-rate material removing chemical reaction. The scanning technique that is used in this system is commonly referred to as a boustrophedon type scan, or an XY bidirectional raster scan.
A boustrophedon type scan operates in two orthogonal directions, i.e. the X-axis and Y-axis coordinate planes. In the material removal system described in the above referenced patent application, the boustrophedon type scan of the chemical etching instrument operates by scanning the probe across the surface of the wafer in a positive and a negative X-axis coordinate plane direction while indexing in a positive or a negative Y-axis coordinate plane direction. Thus, the probe may scan across the wafer surface in the positive X-axis coordinate plane direction, index a predetermined distance in the positive Y-axis coordinate plane direction, scan across the wafer surface in the negative X-axis coordinate plane direction, and then index a further predetermined distance in the positive Y-axis coordinate plane direction. This scan pattern is repeated until the entire surface of the wafer is scanned. Also, since the probe maintains a constant-rate material removing chemical reaction, the scan speed of the probe in relation to the wafer surface is adjusted so as to perform a desired removal of wafer surface material.
Although the boustrophedon type scan permits the material removal system to accomplish its basic objective of scanning the entire wafer surface, the boustrophedon method encounters mechanical problems at high wafer throughput rates and exhibits depreciative etching of surround materials at all wafer throughput rates. The mechanical problems are focused on the indexing, or turnaround, steps of the controlled scan pattern and, to a lesser degree, the scan speed adjustments. The turnaround steps and the scan speed adjustments require a motor to control the precision indexing and the scan speed variations, respectively. Consequently, mechanical stress is inflicted on the motor. Furthermore, as the scan rate is increased, the amount of the mechanical stress inflicted upon the motor is increased. It should be noted that this mechanical stress can be inflicted on either a wafer retention stage motor or a chemical etching instrument motor, depending upon which is used to perform the controlled scan pattern.
The depreciative etching problems are due to the wafer being typically placed on a surround that is secured to a wafer retention stage. The wafer is registered to the surround, and hence to the wafer retention stage, by several tooled surfaces that are secured to the surround. The surround and the tooled surfaces are typically the same material type as the wafer to ensure a consistent chemical reaction between the surface material of the wafer and the plasma. However, a depreciative etching of the surround and the tooled surfaces occurs when the plasma emitting probe is extended outside the edge of the wafer during the turnaround steps. This depreciative etching results in an additional expenditure of capital and labor in replacing the damaged surround materials.
As described above, the use of the boustrophedon type scanning method in a chemical etching process results in both mechanical and depreciated material replacement problems. It is therefore desirable to use an optimal wafer surface scan method that does not result in either mechanical or depreciated material replacement problems in a chemical etching process.